1. Field of the Invention
The present invention relates to a semiconductor device having a display portion. In particular, the present invention relates to a semiconductor device in which thin film transistors are formed over a substrate having an insulation film surface thereon.
2. Related Art of the Invention
Semiconductor devices and, particularly, electronic devices having a semiconductor display unit have been vigorously developed in recent years, and their applications can be represented by such portable devices as game devices, notebook PCs, cellular phones as well as such diversities as liquid crystal TVs, liquid crystal displays, EL displays and so on. As compared to the traditional CRTs, the semiconductor display units can be realized in reduced weights, reduced thickness and consuming electric power in small amounts.
As the conventional semiconductor display units, there have been known a semiconductor display unit of the passive matrix type having a pixel region on which striped electrodes are formed in a manner to intersect each other on the upper and lower sides with the liquid crystal layer or light-emitting layer sandwiched therebetween, and a semiconductor display unit of the active matrix type having a pixel region on which thin-film transistors (TFTs) are arranged like a matrix.
In recent years, technology has advanced for forming TFTs over a substrate, and efforts have been made to apply the semiconductor display unit of the active matrix type. In particular, the TFTs using a polysilicon film exhibit an electric field effect mobility (often called simply as mobility) which is higher than that of the conventional TFTs using an amorphous silicon film, and are making it possible to control the pixels by a drive circuit formed over the same substrate as that of the pixels though the pixels have heretofore been controlled by a drive circuit outside the substrate.
Next, the constitution of an electronic device having a conventional semiconductor display unit will be described. FIG. 21 is a block diagram schematically illustrating portions related to the display of an image. In FIG. 21, a semiconductor device 301 receives or forms image data, processes the image data, converts the format, and displays the image. Examples of the semiconductor device 301 include game devices, video cameras, car navigation systems, personal computers and so on.
On the semiconductor device 301, a semiconductor display unit 302 is constituted by a pixel region 319, a scanning line drive circuit 318 and a signal line drive circuit 317, and is formed as a unitary structure on a substrate having an insulating surface. Other circuit blocks are formed on different silicon substrates and are mounted in the form of IC chips. Some of the circuit blocks may often be formed over the same silicon substrate.
The semiconductor device 301 is constituted by an input terminal 311, a first control circuit 312, a second control circuit 313, a CPU 314, a first memory 315, a second memory 316, and the semiconductor display unit 302. The input terminal 311 receives data that serve as the basis of image data depending upon the kind of the electronic devices. For example, the input data are those through an antenna in the case of a broadcast receiver, and the input data are those from a CCD in the case of a video camera. The input data may be those from a DV tape or a memory card. The data input through the input terminal 311 are converted into image signals through the first control circuit 312. The first control circuit 312 processes the image signals, such as decoding the image data that are compressed and encoded according to the MPEG standard and the tape format, and interpolating and resizing the image. The image signals output from the first control circuit 312 and the image signals formed or processed by the CPU 314, are fed to the second control circuit 313, and are converted into a format (e.g., scanning format, etc.) that is adapted to the semiconductor display unit 302. The second control circuit 313 produces image signals and control signals of which the formals have been converted.
The CPU 314 efficiently controls the signal processing in the first control circuit 312, second control circuit 313 and other interface circuits. The CPU 314 further forms and processes the image data. The first memory 315 is used as a memory region for storing image data from the first control circuit 312 and for storing image data from the second control circuit 313, as a work memory region for executing the control operation by using a CPU, and as a work memory region at the time of forming the image data by the CPU. As the first memory 315, there can be used a DRAM or an SRAM. The second memory 316 stores the color data and character data, and is necessary when the image data are to be formed or processed by the CPU 314. The second memory 316 is constituted by a mask ROM or an EPROM.
The semiconductor display unit 302 is constituted by the signal line drive circuit 317, scanning line drive circuit 318 and pixel region 319. The signal line drive circuit 317 receives image signals and control signals from the second control circuit 313 (clock signals, start pulse signals and the like), and the scanning line drive circuit 318 receives control signals (clock signals, start pulse signals and the like) from the second control circuit 313. The pixel region 319 displays the image.
The electronic device having the semiconductor display unit can assume a variety of constitutions in addition to the constitution shown in FIG. 21. The simplest constitution may comprise the semiconductor display unit, input/output terminals and a simple control circuit as exemplified by a liquid crystal display or an EL display. When the CPU bears a too large load in the architecture shown in FIG. 21, an image processor may be newly provided to reduce the burden of the CPU.
In the conventional electronic device having the semiconductor display unit described above, the circuit blocks other than the drive circuit are mounted being formed over a substrate separate from the substrate over which the pixels are formed.
Accompanying the widespread use of portable electronic devices, it is becoming an important assignment to realize the electronic devices in small sizes. The thus constituted semiconductor devices, however, require many IC chips that are mounted over the substrates separate from the substrate over which the pixels are formed, and involve difficulty if they are to be realized in small sizes. In particular, even if the circuit blocks are realized in small sizes in the IC chip, a large margin required for the mounting makes it difficult to fabricate the entire device in a small size. If it is attempted to decrease the margin for the mounting to realize the device in a small size, then, a high degree of mounting technology is needed arousing a problem from the standpoint of cost and reliability in the mounting portion. There further remains the problem of wiring capacity. That is, when the IC chips are mounted, the wiring must bear a large load making it difficult to conduct the operation at high speeds.
As a method of solving these problems, it has been expected to form the circuit blocks integrally with the semiconductor display unit.
When the circuit blocks are formed over the substrate having an insulating surface, however, a problem often arouses concerning the operation speed. This is because, the TFTs formed over the substrate such as a glass surface having an insulating surface exhibit properties such as mobility and threshold values which are inferior to those of the transistors formed over a single crystalline silicon substrate.
When the conventional semiconductor devices are operated at a given frequency, therefore, a desired operation is realized when the circuit blocks are mounted in the form of IC chips but the desired operation is not realized when the circuit blocks are formed over the substrate having the insulating surface.